fix: colkind uses BBoxMode (#12)

[ithinkandicode]

Closes #12

Checks sepmasks (values here) to set colkind, which sets the sprite collision shape:

• If it's 0 ("AxisAlignedRect"), colkind is set to Rectangle (1), as it's known to be used for certain.
• If it's anything else, colkind is set to Precise (0)

There's no easy way to tell if the collision shape should really be Precise, Ellipse or Diamond, as UMT says they use all Precise sepmasks, but it's a safe default.

For the sake of completion here's the possible values for colkind:

• 0 Precise
• 1 Rectangle
• 2 Ellipses
• 3 Diamond

UMT's "RotatedRect" might mean Diamond, but during tests with GMS 1.4 I didn't see UMT use it.